1. Field of the Invention
This invention relates to the preparation of polymer spray foams and is particularly concerned with a novel catalyst combination for the trimerization of polyisocyanates to polyisocyanurate spray foams.
2. Description of the Prior Art
Rigid polyisocyanurate foams having resistance to flame and heat as well as excellent thermal insulating capacity are known in the art. The prior art discloses methods for preparing such foams by reacting an organic polyisocyanate with a trimerizing catalyst in the presence of a blowing agent, and a minor proportion (usually less than 0.5 equivalent per equivalent of polyisocyanate) of a polyol; see for example U.S. Pat. Nos. 3,516,950, 3,580,868, 3,620,986, 3,625,872, 3,635,848, 3,725,319, 3,745,133, G.B. Pat. No. 908,337, and DBP 1,112,285.
The above methods require the foaming process to proceed in conjunction with at least two polymer forming reactions; namely, the isocyanurate formation arising from the homopolymerization of the isocyanate employed, and the minor amount of polyurethane formation arising from the reaction of a polyol with the isocyanate. Difficulties arise, particularly in commercial applications, due to the variation in relative rates of the two polymerization reactions. The polyurethane formation frequently initiates before the trimerization formation thus giving two separate rise steps in the overall foam rise profile.
These difficulties are accentuated in the case of application of polyisocyanurate foams by spray technique. Accordingly it has hitherto been necessary, when spraying foams of this type, to employ relatively warm substrate temperatures, for example at least 60.degree.F, in order to achieve the rapid foam rise times required and to obtain proper cure and adhesion of the foam to the substrate. The result of simply increasing the concentration of catalysts of the prior art in an attempt to achieve rapid rise characteristics along with good foam cure and substrate adhesion when spraying on cold substrates, is both poor adhesion and foam cure with concomitant loss in foam properties. Because of the critical role which the nature and proportion of catalyst play in the preparation of polyisocyanurate foams, it has not hitherto been possible to devise a catalyst which would perform satisfactorily at low application temperatures and additionally give a foam which possessed good adhesion to the substrate.
We have now found a process which is free from the above difficulties and which is particularly useful in spray foam systems where the polyisocyanurate foam is to be applied at low temperatures to a cold substrate.